The processing of the printed circuit boards generally involves producing a laminated board having an epoxy based substrate with layers of etched copper conductors thereon, and one or more semi-conductor chip components or connectors attached to the board by tiny lead wires. A particularly common configuration for these lead wires is a dual in-line pin, commonly called a D-I-P package. This configuration appears as two rows of electrical leads projecting from opposite edges of the component and bent at 90.degree., such that the leads project downward from the plane of the chip.
Chip components are typically mounted to the printed circuit board by inserting the component leads into holes disposed through the board which extend from the component side of the board to the conductor side of the board. Surrounding each of these holes on the conductor side of the circuit board is a solder pad. A known technique for securing the chip components and connectors to the board by their leads is to mask a portion of the lower side of the board to create a pattern, flux the exposed surfaces, preheat, and then pass the conductor side of the board through a smooth wave of molten solder in a process called wave soldering.
During the wave solder process, molten solder is pumped up and over support plates to form a wave. The printed circuit board is passed over the solder wave by a conveyer at an angle such that the conductor side of the board contacts the leading edge of the solder wave. Component leads are soldered to the solder pads on the printed circuit board and the holes, which are often pre-plated, are filled with solder by capillary action.
If two clean metal surfaces are held together and dipped into the molten solder, the solder will wet the metal and climb up to fill the gaps between the adjacent surfaces. This phenomena is the result of capillary attraction. Wave soldering processes rely on this phenomena to produce a fillet of solder both on the component side of the circuit board and surrounding the portion of the lead which projects through the holes to the conductor side of the circuit board. Unfortunately, however, capillary attraction can produce unwanted solder bridging between closely spaced component leads.
In recent years, the dramatic increase in packaging density of through-hole connectors has led to smaller and smaller lead spacing, which can be as little as 0.050 in (0.127 cm), hereinafter "50 mil". At such spacings, the problem of bridging, especially on the trailing edge of the leads, can be severe.
The art has been replete with many attempts to alleviate the bridging problem. Process parameters such as wave height, conveyers speed, pre-heat temperatures, flux types, and solder temperatures have all been attempted, but have been unsuccessful. For some components, it is still necessary to manually remove the solder bridges by reheating the connections and removing the excess solder with a vacuum device such as "solder sucker" or with a copper braid which wicks molten solder. These are extremely expensive and time-consuming processes and the required remelting can weaken the solder connections. Other techniques have also been developed which have proved promising, but have been relatively ineffective in eliminating bridging in 50 mil components. See Shearer, U.S. Pat. No. 4,339,784, (hereinafter '784), and Haarde, U.S. Pat. No. 4,835,345, (hereinafter '345), which are hereby incorporated by reference.
Shearer '784 discloses a solder draw pad consisting of additional foil pads, or dummy solder pads, arranged in line with the row of leads. The additional foil pad or pads draws off excess solder to reduce the solder bridging between adjacent leads during the soldering operation. The disclosed solder draw pads of this reference are about the same size as the lead solder pads and are spaced in such a way as to form a bridge between the dummy pads and the last row of leads. Although this does not result in a short, such extended pad areas have been known to produce electrical signal distortion or "noise" due to the "antenna effect".
Haarde '345 discloses a robber pad design for receiving and containing excess solder which might otherwise bridge adjacent downstream leads of a component having closely spaced leads. His robber pad is connected to an adjacent upstream solder pad by a solder-wettable bridge. This reference teaches that it is critical that the robber pad or dummy pad be connected by the solder-wettable bridge to the most downstream pad in the linear array of solder pads. Because of the extension of the solder pad, this robber pad configuration is also subject to the antenna effect. Moreover, this design has not proved to be effective in tests for reducing bridging between adjacent leads of high density components.
While in the main such references have provided temporary relief for the bridging problem during some wave soldering processes, there is a need for a more effective technique for solving this problem with high density 50 mil lead spacing components.